A synchronous motor is an AC motor including a rotor and a stator, distinguished by the rotor spinning synchronously with stator frequency. There are two major types of synchronous motors: non-excited and direct-current excited. Reluctance motors belong to the former group.
Before starting a synchronous motor, an initial angle and speed of a rotor of the motor usually have to be known. The initial angle and speed can, for instance, be detected by using a position sensor. The position sensor, however, add costs and unreliability to the system. Therefore, position-sensorless control methods have also been developed.
In a position-sensorless control, the speed and the angle may have to be detected at every start-up. It is known that the initial angle is forced with DC-magnetization (DC-rotation method) or, in the case of salient pole machines, detected with a signal injection method (AC-injection method). Both of these methods can be used in a start-up procedure of a synchronous reluctance motor (SYRM).
A problem associated with the above methods is that they can be utilized on a rotor at a standstill alone, though signal injection methods have also been proposed for detecting rotor speed and angle of a rotating rotor. Start-up methods designed for synchronizing with a rotating rotor of a permanent magnet synchronous machine (PMSM) are generally based on determining a back-EMF voltage of a rotating permanent magnet flux, but no such flux exists in the SYRM.